For selecting the wavelength and manipulating the intensity of the laser light, a laser system in a laser scanning microscope, as described in, inter alia, EP 1 795 938 A2 and DE 19702753C2, has an acousto-optical element (AOTF/AOM), the spectral properties of which are matched to the spectral characteristic of the laser light to be transmitted. FIG. 1 shows, in an exemplary manner, an ideal transfer function H(λ) of an AOTF with a rectangular transducer when excited by a single sinusoidal wave.
The optical transfer properties of the acousto-optical element (AOTF/AOM) are usually set by the cut of the crystal and the geometry of the transducer applied thereon. By selecting the geometry, it is possible here to manipulate both the width of the main lobe and the decay behavior of the side lobes of the transfer function H0(λ). Often, a rectangular transducer is realized, the normalized transfer function H0(λ) of which describes the sinc2 function.
            H      0        ⁡          (      λ      )        =      sin    ⁢                  ⁢                  c        2            ⁡              (                  λ                      λ            0                          )            
For a given laser wavelength λn, the frequency fn of the exciting RF signal is determined, inter alia, by the material properties and the geometry of the crystal. It is inversely proportional to the laser wavelength λn, which may be manipulated by the excited diffraction grating.
      λ    n    ∼      1          f      n      
The transfer function H0(λ) is, inter alia, characterized by the following parameters (see also FIG. 6):                B3dB—bandwidth at which the main lobe of the transfer function has decayed to half of the maximum thereof (also FWHM—full width at half maximum)        BN—spacing of the zeros which delimit the main lobe of the transfer function        
For the purposes of simultaneously manipulating two or more laser lines (λ1, λ2, . . . λn), the RF signal used to actuate the acousto-optical element contains a sinusoidal carrier per laser line. Each sinusoidal carrier with the frequency f1, f2, . . . fn excites a diffraction grating for the corresponding laser line in the acousto-optical element.
On account of the relationship λn˜1/fn, the properties of the transfer function may also be characterized depending on the frequency of the exciting RF signal. In particular, it is possible to specify ΔfBN for the purposes of describing the width of the main lobe in a manner dependent on the detuning of the center frequency of the actuation signal. The following applies:ΔfBN˜1/BN 
For a rectangular transducer, B3dB=0.89 and BN=2 apply. Accordingly, the variables BN and B3dB behave as follows in relation to one another:B3dB=0.445 BN and FWHM=0.445 BN.Conventional Use of the AOTF
The distance Δλ between two adjacent laser lines is selected to be at least so large that the main lobes of the transfer function H0(λ) of the acousto-optical element are not superposed. This is the case if the distance Δλ between two laser lines is not less than the zero spacing BN of the main lobe of the transfer function H0(λ) of the acousto-optical element. The following applies:Δλ≥BN 
Therefore, the following emerges for the rectangular transducer:Δλ≥2.247FWHM
The adaptation of the transducer geometry for adjusting the transfer properties is usually accompanied by a significant increase in the required power of the radiofrequency actuation signal of the AOTF in order to achieve the required diffraction efficiency of >90%. Compared to a standard AOTF with a rectangular transducer, the RF actuation power may have to be raised by up to 7 dB depending on the wavelength of the laser light. Typical values lie at >+27 dBm (˜500mW) for the modified AOTF in comparison with +20 dBm (˜100mW) for a standard AOTF.
Increasing the RF control power causes additional heating of the acousto-optical element and, as a result, causes a change in the optical properties of the crystal (e.g. TeO2) on account of the dependence of the speed of sound on the temperature therein.
There is, in particular, a significant spectral shift in the transfer function H0(λ) as a result of the heating.
It is possible to counteract this unwanted temperature drift by heating the AOTF crystal to a temperature significantly above room temperature (e.g. 50° C.) using a controlled electrical heater (DE-19827140-A1).
Alternatively, it is possible to undertake stabilization of the temperature using a Peltier element or realize temperature-controlled frequency tracking of the RF actuation signal as described in, for example, DE202007015506 U1 and DE19827140 A1.
Local changes in the crystal temperature as a result of brief variations in the RF actuation power—for example due to an intensity modulation of the laser light while recording the image—cannot be captured and compensated by such methods and have as a consequence unwanted changes in angle and intensity of the laser light in relation to the first order of diffraction of the AOTF as a result of the spectral shift of the transfer function H0(λ).